Yes this threw me off when I first opened it. When you select the letter field you see a flashing cursor but you can't erase the character and type a new one. Instead you have to type a new character which overwrites the old "editing" character. This is confusing because it's not the behavior that the user expects when they see a flashing cursor in a text input.

I would suggest allowing people to erase the letter and type a new one in the editing field.

Once I figured out this little UI hiccup I found it absolutely delightful to play with this. What a fascinating experiment in making a font immediately editable, like a mini font-REPL. I've often been interested in (but never dabbled in) creating or editing fonts. This made that itch immediately scratchable in a raw, primitive way that unlocked something interesting in my brain.

This is a great example of what art can do.